1. Field of the Invention
This invention relates to x-ray imaging systems. In a primary application the invention relates to diagnostic x-ray systems where selected images are created representing specific body materials.
2. Description of Prior Art
In U.S. Pat. No. 4,029,963 issued to R. E. Alvarez and A. Macovski a method was described of acquiring and processing x-ray images at two energy spectra. The processed images can subsequently be used to selectively image various body materials. In the referenced patent two photosensitive emulsions are exposed using two scintillating screens. One problem with the system shown is that of achieving sufficient separation of the two energy spectra used to obtain the two acquired images. When these spectra have significant spectral overlap, with relatively small separation in the average energies, the resultant processed images have poor signal-to-noise ratio.
Another problem with the method shown in that patent is that of registration. In the system shown, the films representing each energy spectrum are scanned independently. This can often result in misregistration with the processed images becoming distorted. This registration problem was solved in pending patent application Ser. No. 299,208, filed Sept. 3, 1981 "X-Ray Encoding Systems Using an Optical Grating" invented by R. E. Alvarez, A. Macovski and B. Strul. In that application the information is encoded using an optical grating. Here the two images are represented by different spatial frequencies and can thus remain perfectly registered. In this system, however, the readout operation requires resolving the very high spatial frequencies of the optical gratings. Thus much of the resolution capacity of the optical scanner is used for the grating frequencies.